A compressed air dryer is used for removing water vapor from compressed air. Compressed air dryers are commonly found in a wide range of industrial and commercial facilities. The process of air compression concentrates atmospheric contaminants, including water vapor. This raises the dewpoint of the compressed air relative to free atmospheric air and leads to condensation within pipes as the compressed air cools downstream of the compressor.
Excessive water in compressed air, in either the liquid or vapor phase, can cause a variety of operational problems for users of compressed air. These include freezing of outdoor air lines, corrosion in piping and equipment, malfunctioning of pneumatic process control instruments, fouling of processes and products, and more.
There are various types of compressed air dryers. Their performance characteristics are typically defined by the dewpoint. The primary types of dryers are regenerative desiccant dryers, often called “twin tower” dryers; refrigerated dryers; deliquescent dryers; and membrane dryers.
The most common moisture removal technology employed with on-site medical air application is heatless regenerative desiccant dryers. These dryers typically comprise of two vessels charged with activated alumina (silicon), and are operated on a timed switching basis whereby one dryer removes moisture from the process air whilst the other dryer purges to regenerate. When the preset time interval is reached, the process air flow switches from the active tower to the stand-by tower and the active tower purges to remove accumulated moisture. Typically, a portion of the dried air (5-15%) is used to back-flush the off-line dryer.
While this operating method does a good job of removing moisture from the process air, the volume of purge air required places unnecessary demand on compressor systems during periods of low demand for the process air. To reduce power consumption and limit compressor wear, most desiccant dryers deployed for the medical air application are designed to control purging based upon measured dewpoint. In this case, the purge valve on the stand-by tower remains closed until the process air dewpoint reaches a pre-determined level. The towers continue to switch on a timed basis, thus loading with moisture equally until the purge activation dewpoint is reached and the purging cycles begin.
A previously unrecognized side effect of this desiccation process has been identified by the inventor. It has been discovered that traditional desiccant dryer operating methods contribute to variable CO2 levels within the process air, and in certain cases measurements indicate CO2 levels exceeding the United States Pharmacopeial Convention (USP) limit of 500 ppm. Therefore, there is a need in the industry to address the need to limit both dewpoint and CO2 levels in dried medical air applications.